1. Field of the Invention
Embodiments of the invention relate to a wafer stage. In particular, embodiments of the invention relate to a wafer stage adapted to hold a wafer, and a method for attaching a wafer to a wafer stage.
This application claims priority to Korean Patent Application No. 10-2006-0000652, filed on Jan. 3, 2006, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
In general, semiconductor devices are fabricated by application of a complex sequence of fabrication processes (e.g., material deposition, etching, ashing, diffusion, photolithography, etc.) to a wafer. By means of these selectively and repeatedly applied fabrication processes, various material layers (e.g., conductive and insulating layers) are formed and patterned on the wafer.
Among the fabrication processes commonly used to manufacture semiconductor devices, photolithography processes generally comprise a coating process wherein a wafer is coated with a photoresist; an exposure process during which the coated wafer is selectively exposed to light through a reticle defining a circuit-pattern; and, a developing process during which a developing solution is applied to remove the portions of the photoresist from the wafer.
Conventional photolithography equipment adapted for use in photolithography processes may be generally divided into coating/developing equipment (e.g., a spinner) and exposure equipment (e.g., a scanner or stepper). Spinners and scanners are typically associated with one another in a fabrication line to perform photolithography processes.
Conventional exposure equipment includes a wafer stage adapted to hold a wafer. The wafer stage typically includes a wafer chuck adapted to directly hold the wafer, and a driver adapted to move (i.e., transfer) the wafer chuck.
The wafer chuck includes lift pins on a central portion of the wafer chuck, and the lift pins may be raised and lowered so that the wafer may be placed on the wafer chuck. The wafer chuck also includes a plurality of vacuum holes adapted to draw the wafer onto the wafer chuck once the wafer is positioned by operation of the lift pins.
That is, to expose a wafer coated with photoresist, for example, the wafer is first positioned over the wafer chuck by a transfer robot. The lift pins of the wafer chuck are raised to support a bottom surface of the wafer. The wafer, which is supported by the lift pins, is then positioned on a top surface of the wafer chuck as the lift pins are lowered. As soon as the lift pins have been completely lowered, the wafer is held to the wafer chuck by a vacuum suction force provided through the vacuum holes of the wafer chuck. The wafer, which is held to the wafer chuck by the vacuum suction force, is held (i.e., fixed) to the wafer chuck with a predetermined amount of pressure so that the wafer will not shake during the exposure process for forming a predetermined pattern on the wafer.
However, as illustrated in figures (FIGS.) 1A and 1B, a conventional wafer stage 10 comprises lift pins 30 disposed in a central portion of a wafer chuck 20, as described above. Thus, a wafer W, and in particular wafer W having a diameter of 300 mm, which is placed on wafer chuck 20 by lift pins 30 and held to wafer chuck 20 using vacuum holes (not shown), sags at its edge of when it is placed on wafer chuck 20 to be held by wafer chuck 20. As illustrated in FIG. 1B, wafer chuck 20 collides with the edge of wafer W when wafer W is placed on wafer chuck 20. Thus, placing wafer W on and attaching wafer W to conventional wafer stage 10 may damage the edge of wafer W. Further, when the exposure process is performed for a relatively long amount of time, a portion of wafer chuck 20 may be worn down by periodic collisions between wafer W and wafer chuck 20.